Spray-applied ceramic fiber insulation

ABSTRACT

A thermal insulation is formed by simultaneously spraying ceramic fibers and a binder at a surface to be covered. An aqueous binder is prepared prior to application to the fiber during the spraying operation. The binder includes an organic component which provides necessary wet and green adhesion to the substrate surface and wet and green cohesion to the newly formed lining prior to firing of the sprayed-on layer. The binder also includes an inorganic component which functions at and after exposure to a high temperature to secure the ceramic fibers to the substrate and to one another. Drying and firing of the layer develops strength in the inorganic binder. Known ceramic fibers such as alumina-silica amorphous fibers, alumina-silica-zirconia fibers, polycrystalline mullite fibers, alumina fibers, mineral fibers, or combinations thereof, and the like may be employed. The fiber may be precoated with the organic binder. Preparation of the binder, coating of the fibers with an organic binder component and spray formation of the ceramic fiber insulation are described.

The present invention relates to mixtures of ceramic fibers and anorganic-inorganic binder system. More particularly. the presentinvention relates to a sprayable ceramic fiber, thermally-insulatinglayer, precoated fibers therefor and their formation and binder systemssuitable for use in spray application of ceramic fibers to form athermally insulating layer.

BACKGROUND OF THE INVENTION

Insulating fiber linings sprayed-over refractories have been describedat pages 42-44 of "Industrial Heating," November, 1984 issue. Thisreference discloses a sprayed-on thermally insulating fiber liningcomposed of a mixture of alumina-silica fibers and an inorganic binderof composition not disclosed in this article. A later publication by thesame company (Manville Service Company) indicates the composition of asuitable binder to be acid aluminum phosphate.

A commercially available ceramic fiber binder is sold under the name"Strataseal™" by Strataseal Corporation McMurray, Pa. While the specificcomposition of this binder has not been revealed, this material is knownto be toxic and highly acidic. When used, precautions must be taken toavoid inhalation of its fumes or fibers coated therewith or contact ofit or coated fibers while wet with the skin. A further disadvantage ofthis system is revealed subsequent to its application to a steel furnacesubstrate. With passage of time, corrosion of the metal substrate hasbeen observed, which corrosion is believed due to the acidic characterof the this binder system which is believed to contain, inter alia,phosphoric acid. A further problem with known spray-on ceramic fiberinsulation systems is a lower level of wet adhesion and wet cohesionthan is desired and/or necessary in many situations. The term "adhesion"as used herein is in reference to adherence of the ceramic fibers of alining to a substrate such as a furnace surface. The term "cohesion" asused herein is in reference to coherence of the ceramic fibers of alining to one another. The terms "wet adhesion" and "wet cohesion" asused herein are in reference to adherence and coherence respectively ofthe ceramic fibers while the vehicle of the binder system is present onthe fibers. As the lining thickness builds up, its weight does also.When applied to ceiling surfaces the weight of the newly installed wetlining commonly causes separation of the newly applied lining from itssubstrate. While this problem can be ameliorated by mechanical deviceswhich are affixed to the substrate prior to application of the sprayablelining, such practice is not preferred due to additional materials andcosts and reducton in thermal insulation efficiency. Such mechanicalsupports means may include the installation of metal mesh in spacedrelationship to ceiling or overhead surfaces, or stud anchors which areaffixed to the substrate and protrude in the direction of the liningthickness away from the surface of the substrate.

It is thus a primary object of the present invention to reduce oreliminate toxicity hazard associated with prior sprayed-on ceramic fiberinsulation systems. It is also an object of the present invention toimprove the wet adhesion and wet cohesion of the newly sprayed-on layerso that greater thicknesses of lining may be applied to an overheadsurface with minimum need of mechanical support means. It is also anobject of an embodiment of the invention to provide an inorganic-organicbinder which facilitates handling and spraying of the ceramic fiber andbinder.

SUMMARY OF THE INVENTION

According to the present invention, there is provided a thermallyinsulating layer comprising from about 72.5 to about 97.5 weight percentof ceramic fibers, sufficient organic binder component to impart aminimum of about 0.2 g/cm² (0.41 lb/ft²) adherence while wet to theintended substrate, and from about 2.5 to about 30 weight percent of aninorganic binder component derived from colloidal high temperatureresistant refractory oxides.

The necessary adherence to the substrate depends upon density andthickness of the layer. The amount of adhesion and cohesion must besufficient to offset the combined weight of the ceramic fiber, binderand binder vehicle. For example, only about 0.2 g/cm² is the necessaryadhesion for a 4 lb/ft³ (dried density) and 1 inch thick lining whereasabout 3.0 g/cm² is necessary for a 10 lb/ft³ (dried density) and 6 inchthick layer which may have a wet weight of about 20 lb/ft³. Thenecessary wet adhesion and wet cohesion is obtained by an organicpolymer component of the binder. From about 0.05 to about 2.0 weightpercent based on ceramic fiber weight when utilizing standard orlubricated fiber, and to about 5 weight percent when precoated fiber isutilized, of a water-soluble organic polymer containing polar groups,preferably selected from the group including acrylamides,hydroxyethylcelluloses, carboxymethylcelluloses andcarboxyethylcelluloses has been found to be effective in imparting thenecessary wet adhesion and wet cohesion. Increasing the amount oforganic binder component increases the amount of wet adhesion and wetcohesion.

According to another aspect of the present invention, there is provideda method of forming a thermally-insulating layer on a substratecomprising providing precoated ceramic fibers having a coating thereonof organic binder component sufficient to impart a minimum of about 0.2g/cm² adherence to steel or the chosen substrate of interest; providinga binder including an aqueous solution/dispersion having from about 1.5to about 40 weight percent of colloidal high temperature resistantoxides; applying said binder to said precoated ceramic fibers whilespraying said fibers to impact toward the substrate. The binder may alsoinclude organic binder component in solution.

According to a further aspect of the present invention. there isprovided a method of forming a thermally insulating layer on a substratecomprising providing ceramic fibers, providing aqueous binder comprisingsufficient organic binder component to impart a minimum of about 0.2g/cm² adherence to steel or the chosen substrate of interest, and fromabout 2.5 to about 30 weight percent of colloidal high temperatureresistant refractory oxides; applying said binder in amount of fromabout 75 to about 180 weight percent based on fiber weight to theceramic fibers while spraying the fibers toward the substrate.

BRIEF DESCRIPTION OF THE DRAWING

In the drawing,

FIG. 1 illustrates an apparatus for determining the adherence value ofbinder compositions in accordance with the invention.

FIG. 2 is a schematic illustration of a process and apparatus used informing precoated ceramic fiber in accordance with the invention.

DETAILED DESCRIPTION OF THE INVENTION

As used herein, the terms "having," "including," "comprising" and"containing" are synonymous. Unless otherwise specified at the point ofuse, all percentages, fractions and ratios given in this specificationand the appended claims are on a weight basis. "Green," when used incombination with or in reference to a physical property, e.g., adhesion,cohesion or strength, refers to that physical property of the sprayed-onceramic fiber lining or its components when dried to remove thesolvent/vehicle of the binder but not yet fired at or above thedecomposition temperature of the organic binder component. "Rebound," asused herein, refers to sprayed material which does not adhere to thesurface at which the sprayed material is directed but rather rebounds orbounces away. Rebound is an undesired phenomenon.

MATERIALS

The first component of the refractory sprayable, thermally insulatingcomposition of this invention is ceramic fiber, which is present inamount of from about 72.5 to about 97.5 percent by weight. The ceramicfiber is preferably selected from the group consisting of fibers ofalumina-silica, alumina-silica-zirconia, polycrystalline mullite fibers,calcium-alumino-silicate, alumina, mineral fibers and the like. Theparticular fiber is chosen dependent upon the temperature andatmospheric conditions anticipated in service in a manner well known tothose skilled in the art of high temperature thermal insulation usingceramic fibers.

Of the above-given classes of fibers, those of alumina-silica andalumina-silica-zirconia, such as those sold by Sohio EngineeredMaterials Company (SEM) of Niagara Falls, N.Y., under the trademarkFiberfrax® ceramic fibers are preferred for installations where thecontinuous use temperature will not exceed 1427° C. (2600° F.). Whenhigher service temperatures are contemplated, Fiberfrax®alumino-silicate ceramic fibers may be admixed with Fibermax™polycrystalline mullite fibers which are available from Sohio EngineeredMaterials Company of Niagara Falls, N.Y. When polycrystalline mullitefibers alone are employed, continuous service temperatures may be ashigh as 1649° C. (3000° F.).

The manufacture of alumino-silicate refractory fibers is described inU.S. Pat. No. 2.557.834. The manufacture of alumina-silica-zirconiarefractory fibers is described in U.S. Pat. No. 2,873,197. Themanufacture of polycrystalline oxide fibers of, for example,alumino-silicate, is described in U.S. Pat. Nos. 4,159,205 and4,277,269. A particularly preferred ceramic fiber for use in the presentinvention is Fiberfrax® bulk fiber which is an alumino-silicate ceramicfiber having a continuous service temperature upper limit of about 1260°C. (2300° F.), and a mean fiber diameter of 2-3 microns.

The method of manufacture of the ceramic fibers is not critical. Fibersproduced by blowing, spinning, sol-gel and other methods may be used information of spray-applied layers according to the present invention.

The ceramic fibers may be refined to remove the shot which naturallyoccurs during formation of such ceramic fibers by blowing or spinning ofa molten stream of ceramic material with a high velocity stream of airor rapidly rotating wheels respectively, but removal of the shot is notnecessary prior to usage of fibers in the present invention. Use ofshot-free fiber results in sprayed-on layer of lower density. Use ofshot-free fiber results in sprayed-on layer of greater thermalinsulation value than one of shot-containing fiber at equal layerdensity.

Length of the ceramic fibers is not critical. Fibers of a length whichcannot be readily handled by the particular fiber spraying apparatusemployed may be chopped into reduced lengths to facilitate theirdelivery from the spraying apparatus. Generally, use of fibers ofgreater length results in sprayed-on linings of lower density and viceversa.

Diameter of the ceramic fibers is believed to not be critical.Typically, commercially available ceramic fibers sold for use as thermalinsulation range in diameter from about 2 to about 5 microns. All aredeemed suitable for use in the present invention. Larger diameter fibersresult in a spray-applied layer exhibiting less thermally inducedshrinkage.

In a particularly preferred embodiment of the present invention, theceramic fibers are lubricated at the time of their manufacture tofacilitate their separation and conveyance into and through the sprayingapparatus. Equipment presently available is capable of handling fiberlengths up to about 2 inches. Greater lengths result in clogging of thespraying apparatus.

When bulk alumino-silicate fibers are employed, a particularly preferredlubricant is 50 HB-100 polyalkylene oxide available from Union CarbideCorporation, applied at from about 0.25 percent to a maximum level ofabout 1.0 percent and preferably about 0.5 percent based on weight ofthe fiber. This material is a 50 percent ethylene-oxide, 50 percentpropylene-oxide.

In preferred embodiments of the invention, the fiber at the time of itsmanufacture is coated with an organic polymer binder component to bedescribed in detail herein below. This polymer serves to impart wetadhesion and wet cohesion to the sprayed-on ceramic fibers duringformation of a sprayed-on layer. It serves as a binder and tackifier tothe wet layer. This polymer continues to serve as a binder upon dryingof the sprayed-on layer until the polymer's maximum temperature isexceeded, upon which it decomposes and the ceramic fibers are thereafterbound to one another by an inorganic binder. Fiber-to-fiber ceramicbonding through sintering also occurs if temperatures and exposure timeare sufficient.

A second component of the refractory insulating sprayable composition ofthis invention is a binder system. The binder system includes twocomponents: an organic component and an inorganic component. The organiccomponent functions at temperatures below its decomposition temperatureas a tackifier and adhesion-enhancing agent to adhere the newlyspray-applied wet fibers to one another and to the substrate, whether ofmetal or refractory material. The organic component reduces the amountof rebound. When the substrate is porous and absorbent, migration of thebinder to the substrate occurs which contributes to failure of theinsulating layer through loss of adhesion/cohesion. Migration of thebinder to the hot face of the sprayed-on layer can also occur when a newspray-applied layer is dried resulting in a hard crusty layer of ceramicfibers at the hot face. The organic component reduces inorganic bindermigration along and from the ceramic fibers. The "hot face" is thatwhich is presented to the hot interior of a furnace kiln, oven, etc. Theuse of an organic component enables laying up in a single sprayapplication a layer of greater thickness on overhead surfaces than ispossible without usage of such a binder component when all other aspectsare held constant and thus reduces need for mechanical supports. Theinorganic binder component serves to bond the fibers to one another andto their substrate at temperatures above the decomposition temperatureof the organic component. The inorganic component to a small extent alsocontributes to wet adhesion and wet cohesion of the ceramic fibers.

Suitable as the organic binder component is an organic polymer substancecontaining polar groups known to impart tackiness or adhesive propertiesto the polymer. Particularly suitable polar groups are amide, ether,hydroxyl, carboxyl, urethane and amine groups. Preferred are thoseselected from the group consisting of acrylamides,hydroxyethylcelluloses, carboxymethylcelluloses andcarboxyethylcelluloses. Mixtures of suitable organic materials also maybe used. The organic component may be anionic, non-ionic or cationic,the choice being made with reference to the inorganic component toassure compatability and avoid precipitation prior to application of thebinder to the fiber. Generally, non-ionic organic component may be usedwith any of the recommended inorganic binder components; anionic organicbinder component should be used with anionic inorganic component; andcationic organic binder component should be used with cationic inorganicbinder component.

The organic polymeric binder component is applied to the ceramic fibersin amount sufficient to impart a minimum of 0.2 g/cm² wet adhesion tothe substrate. An amount of from about 0.05 to about 5.0 weight percentand preferably about 0.5 weight percent is applied based on weight ofthe fiber.

Of the aforementioned organic binder components is preferred an anionicwater-soluble/dispersable acrylamide-based polymer available under thetrademark Reten® 525 from Hercules, Inc. This acrylic polymer providesexcellent adhesion of the coated fibers, when wetted, to each other andto the surface of the substrate toward which they are sprayed. Othersuitable organic binders include a hydroxyethylcellulose (non-ionicpolymer) available under the trademark Natrasol® 250 HHR from Hercules,Inc., Methocel® A4M methylcellulose (non-ionic) water soluble from DowChemical Company, acrylamide based copolymer water soluble (low cationicactivity) Reten® 210 from Hercules, Inc., acrylamide-based copolymerwater soluble (moderate cationic activity) Reten® 220 from Hercules,Inc., acrylamide-based polymer water soluble (non-ionic) Reten® 420 fromHercules. Inc., and water soluble acrylic polymer Carbopol® from B. F.Goodrich Chemical Company, and the like.

As previously stated, it is preferable that the organic binder componentbe applied to the fiber during manufacture of the fiber. This may bedone by dissolving or dispersing a finely divided powder of the organicbinder in water and introducing it into the gas stream which is utilizedto convert the molten stream of ceramic material into fibers. As shownin FIG. 2, the organic binder component is preferably applied by feedingthe finely divided powder 28 via a twin auger feeder 30 mounted abovethe primary high velocity air stream 31 from primary air nozzle 32.Primary air stream 31 containing lubricant solution 34 impacts thestream 33 containing the molten ceramic material stream 33 to befiberized. The lubricant solution is introduced into the primary airstream which impacts the molten ceramic material. The lubricant solutionsufficiently wets the dry powdered organic binder component to cause itsparticles to become sufficiently tacky so the organic binder componentadheres to the newly formed fibers 36. If no lubricant is being applied,the organic binder is fed into water alone. Although the molten stream33 that is fiberized is at a temperature of about 1982° C. (3600° F.)when alumino silicate fibers are being formed from kaolin, it has beenfound that little or no degradation of the organic binder occurs due tothe rapid cooling of the molten stream 33 as it is fiberized.Application of the organic binder component to the fibers 36 duringtheir formation permits usage of higher levels of organic bindercomponent and hence results in higher levels of wet adhesion, wetstrength and green strength of the newly formed sprayed-on ceramic fiberinsulation layer on a substrate. The organic binder can also be added asa solution/dispersion with lubricant via secondary air stream 37 whichis introduced through secondary air nozzle 38 to extend in length andattenuate in diameter the ceramic fibers 36 as they are being formed.Application of the organic binder component can also be done during theceramic fiber spinning process by introducing the organic component withthe lubricant system. The precoated ceramic fiber is dry to the touchand is not tacky until wetted with a solvent for its coating of organicbinder component. When the organic binder component is applied duringceramic fiber manufacture, the actual amount imparted to the ceramicfiber may be somewhat less than is determined by calculation based onceramic fiber material and organic binder feed rates.

While not preferred, it is also possible to apply the organic binder tothe ceramic fiber subsequent to its manufacture. This may beaccomplished by dissolving the organic binder component in water, eitherseparately or in combination with the inorganic binder component. It ispreferred to form a pre-mix of concentrated organic binder componentwhich is thereafter combined with the inorganic binder component toobtain the desired overall concentration of the organic and inorganiccomponents. In this latter embodiment, the amount of organic binderwhich can be conveniently introduced into the liquid is less than thatwhich can be applied to the fiber at the time of fiberization;therefore, higher wet adhesion and wet cohesion of the ceramic fiberscan be obtained with a precoated fiber. This is due to the fact that atdesired concentrations of suitable organic binder components, theirwater solutions are very viscous and gelatinous. For example, only about0.1 weight percent of Reten® 525 in ordinary tap water results in aviscous and gelatinous solution. These aqueous solutions of organicbinder components are difficult to pump to the spray apparatus and toatomize into a spray for effective coating of the ceramic fibers as theyare simultaneously delivered from the spraying apparatus and directed atthe substrate to be insulated.

The inorganic binder component functions to secure the ceramic fibers toone another and to the substrate when dry (green adhesion and greencohesion) and when in service at temperatures above the decompositiontemperature of the organic binder component. The inorganic componentalso imparts wet adhesion to a small extent. A necessary ingredient ofthe binder system for the present invention is from about 2.5 to 30weight percent solids (dry weight basis) derived from finely divided(colloidal) sol of an inorganic high temperature refractory material. A"sol" is a stable dispersion of discrete colloid size particles inaqueous media. Sols have extremely small particle size and large surfaceareas. Suitable sols include those of silica, alumina, zirconia and thelike, and mixtures of these. Of these, silica is preferred for mostapplications unless specific resistance to chemical attack or higherservice temperature indicates a need for a different material.

Colloidal silica sols may be acidic or basic in nature. Preferredbetween these types is the basic-type material because the corrosioninduced in a metal substrate, for example, a steel lining of a furnace,is minimal when contrasted with that following use of acid-type binder.An amorphous colloidal silica having extremely small particle size, e.g.on the order of 12 to 15 millimicrons and large surface area, isdesired. One of the colloidal silicas preferred for use in compositionsof this invention is Ludox® HS-40 colloidal silica, a basic-type productof E. I. Du Pont de Nemours Company, which has a pH of about 9.7. Thisproduct is an aqueous dispersion of 40 percent amorphous silica, thesilica particles having an average size of 13-14 millimicrons, thesuspension having a density of 10.8 pounds per gallon corresponding to aspecific gravity of 1.28. Another suitable colloidal silica sol isNalco® 1034-A, an acid-type product of Nalco Chemical Company, which hasa pH of about 3.2.

Other suitable inorganic binder components include colloidal alumina,i.e., Nalco® ISJ-614 from Nalco Chemical Company, Oakbrook, Ill. andcolloidal zirconia. i.e., zirconia sol-acetate stabilized from RemetCorporation, Chadwicks, N.Y.

The inorganic binder component is prepared for use in the presentinvention by diluting the colloidal sol with water to form an aqueousdispersion having from about 1.5 to about 40 percent (dry weight basis)colloidal solids and preferably from about 5 to about 15 percentcolloidal solids and, more desirably. about 10 percent solids. Theamount of inorganic binder solids desirably imparted to thespray-applied ceramic fiber based on weight of the fiber is from 2.5 to30 weight percent. Ordinary tap water may be used as a diluent vehicle.When too little inorganic binder component is employed, the shrinkage ofthe sprayed ceramic fiber lining will be larger than when greateramounts are employed. Also less than maximum wet adhesion and cohesionoccur. As the amount of inorganic binder component is increased beyondthe recommended range, the resultant ceramic fiber sprayed-on insulationtends to become too crusty and friable to meet industry performancerequirements or expectations. Higher concentrations of inorganic bindercomponent within the previously specified range are recommended when thefurnace burner exhaust velocity is high to reduce or eliminate erosionof the sprayed-on ceramic fiber insulation, or when mechanical shock ispresent as in a removable insulated cover.

While not preferred due to convenience, environmental, worker safety andcost considerations, it is possible to utilize other liquid vehicles toaid in solution/dispersion of carriers for the organic and inorganicbinder components. The liquid vehicle should be chosen so that it is anon-solvent for the ceramic fiber while a solvent for the organic bindercomponent. The liquid vehicle does not have to fully dissolve theorganic binder component; rather, it is sufficient if the organiccomponent be tacky when wetted with the liquid vehicle. Water is highlypreferred as a liquid vehicle but other liquid vehicles can also be usedsuch as methanol, ethanol, ethylene glycol, diethylene glycol andmixtures thereof, including mixtures with water. Such mixtures can beused to enable utilization as organic binder components materials whichwould otherwise be unacceptable due to their low solubility in wateralone and therefore low resultant wet adhesion and wet cohesion valueswhen employed with water alone. Stable emulsions and dispersions oforganic binder component are also within contemplation of the presentinvention.

LABORATORY BINDER ADHESION TESTING

In order to effectively and economically screen potentially suitablebinder compositions for use in the present invention, a laboratory testwas designed to measure wet adhesion of ceramic fibers coated withbinder. The apparatus used to determine adhesion is illustrated in FIG.1 of the drawing. The apparatus illustrated is used in conjunction witha conventional movable crosshead tester (not illustrated), such as oneavailable from Instron Corporation. Adhesion is determined by measuringthe force required to separate the end of sample 12 of ceramic fiberblanket of known cross-sectional area from anchored substrate plate 22.

The lower end 13 of sample 12 is wetted with the binder candidate to beevaluated and thereafter contacted to the substrate plate 22. The forcemeasured is transmitted through sample 12 and binder 24 to plate 22.Binder is applied by quickly dipping into and quickly removing the lowerend 13 of the sample 12 from a vessel containing bindersolution/dispersion. After wetting with binder 24, the lower end 13 ofsample 12 is perpendicularly contacted to plate 22 and allowed to remainin contact therewith for 30 seconds to allow the binder to set beforeapplying a vertical force 26 to separate sample 12 from plate 22.

Dimensions of sample 12 and apparatus 10 are not critical. Jaw members20 include needles 21 secured thereto which penetrate sample 12 andassist in retaining sample 12, particularly when positioning a sampleinto apparatus 10. Upper horizontal bar 14 is adapted to be connected tothe moving crossbar of the testing machine. Lower crossbar 16 is joinedto upper crossbar 14 by slidable links 18. Jaw members 20 are slidablyretained by lower bar 16. For convenience, jaw members 20 are of alength greater than the width of sample 12 and one jaw member isprovided with a threaded aperture at each end while the other jaw memberis provided with an aperture at each end which can be aligned with itscorresponding threaded aperture. When machined appropriately, ordinarycapscrews are employed to force jaw members 20 toward each other andthereby compressibly engage and retain sample 12.

To obtain the adhesion values reported herein, samples of SohioEngineered Materials Company blanket (Durablanket™) having a nominaldensity of 4 pounds per cubic foot (pcf), a nominal thickness of 1 inch(2.5 cm) were used. Blanket samples of 6 inch (15 cm) width by 3 inch(7.5 cm) vertical length by 1 inch (2.5 cm) were prepared. Anchor plate22 was formed of metal, e.g., steel or aluminum and was thoroughlycleaned prior to each test. Aluminum is preferred for plate 22 becauseit does not rust. All other parts of apparatus 10 are preferably formedof steel.

SPRAY APPLICATION EQUIPMENT

Spraying apparatus suitable for use in practice of the present inventionis available from Unisul Inc. of Winterhaven, Fla., under the name"Poly-Spray A-300 Fiber Spray Machine." Model A-300-5-47 was utilized inthe examples discussed hereinbelow. Other fiber spraying machines mayalso be used. Unisul Inc.'s Unifan™ nozzle is recommended. This nozzlehas a plurality of binder delivery spray nozzles equidistantly arrangedabout the periphery of its race track-shaped delivery end. The centralportion of the race track is the port through which the dry ceramicfiber is delivered. In accordance with a preferred embodiment of thepresent invention, the middle binder liquid nozzle on each long stretchof the race track is replaced or repositioned such that its liquid spraypattern is directed to form two flat fan patterns which convergeapproximately 2.5 cm (1 inch) beyond the delivery face of the nozzle.The Unifan™ nozzle, as normally supplied. delivers the liquid bindersolution from nozzles which are arranged to throw a spray pattern whosefan shapes are generally perpendicular to the exit face of the nozzle.

INSTALLATION OF SPRAYED-ON CERAMIC FIBER LINING

The surface of the substrate to which ceramic fiber layer is to bespray-applied is prepared to receive the layer. Preferred preparationvaries according to the type of substrate to which the spray-appliedlayer is to be applied.

When the substrate is a metal surface, e.g. steel, the surface should befree of all oily substances and rust. Blast cleaning is a highlyeffective method. Surface temperature should be at least 10° C. (50° F.)at the time of spray application. Mechanical supports, when used, shouldbe securely attached at sufficient intervals such that they do notrestrict or prevent the sprayed ceramic fiber from impacting directly onthe substrate. For example, 1/2 inch opening mesh expanded metal, whenpositioned parallel to the substrate surface. is not a desirable supportmaterial because it does not allow complete penetration of the fiber.

When installing a spray-applied layer according to the presentinvention, corners and joint areas should be sprayed first. Wheremechanical supports are used, the areas behind these supports should besprayed first to assure proper and complete packing of the fiber aroundthe supports. This technique also maximizes transfer of load from thesprayed-on layer to the supports. The layer should be built up graduallyto the desired thickness. Multiple passes should be used for each givenarea to be insulated. Approximately one to 2 inches (2.5 to 5.0 cm)should be applied in a single pass, even though much greater amounts canbe applied. Application of too much material in a single pass may resultin separation of the wet layer from the substrate, especially on ceilingor overhead horizontal surfaces. Overhead downwardly facing orhorizontal surfaces should be sprayed first and thereafter verticalsurfaces such as walls. The Unifan™ nozzle should be held about two tothree feet (two thirds to one meter) from the surface being covered.Delivery should be perpendicular to the surface to be covered wheneverpossible to minimize rebound.

When the substrate is a non-metallic refractory material, its surfaceshould be clean and free of all carbon, oxide and glassy deposits. Allloose deposits should be removed. Blast cleaning is a highly effectivemethod; wire brushing is also effective. Mechanical supports arerecommended on overhead non-vertical surfaces including arches, inclinedand horizontal surfaces. The temperature of the surface should be atleast 10° C. (50° F.). Many refractory surfaces are porous, e.g. densebrick; these surfaces should be presprayed with the binder andsufficient time allowed for the binder vehicle to be absorbed beforespraying of the fiber begins. When ceramic fiber is to be sprayed overlow density brick which is very porous, e.g., IFB (insulating firebrick), the brick should be presprayed with the binder sol immediatelybefore beginning application of the fiber. When the substrate is lowdensity brick, the prespray should not be allowed to dry beforeapplication of the fiber begins. The fiber spray procedure is as givenabove for metal. If the ceiling is of the sprung arch type, the sectionat the highest portion of the arch should be the first part sprayed withfiber.

Where even greater wet adhesion and wet cohesion appear to be needed,e.g., high absorbent porous brick, precoated ceramic fiber and liquidbinder containing inorganic and organic components may be used together.

After spray installation, when possible, the lining should be fired to427° C. (800° F.) for about 8 hours to cause slow removal of the bindervehicle, curing of the organic component and removal of the organiccomponent, and then fired to the maximum normal operating temperature.When fired to about 982°-1038° C. (1800°-1900° F.) for about 24 hours,ceramic bonding of the fibers occurs.

A furnace, kiln, or a unit or surface requiring thermal insulation, maybe insulated according to the present invention, as shown in thefollowing examples.

EXAMPLE 1

PRIOR ART BINDER SYSTEM.

A refractory brick-lined car bottom annealing furnace was sand-blastedprior to application of sprayed-on ceramic fiber insulation. Thissurface was irregular due to spalled brick. Bulk lubricatedalumino-silicate fiber (Sohio Engineered Materials Co. Fiberfrax® 6000L)was utilized. The binder was prepared by combining approximately twogallons of concentrated Strataseal™ binder with water to form about 55gallons of solution. The Unisul™ spray equipment was adjusted to deliverceramic fiber at a rate between 8 and 10 pounds per minute and binder ata rate between 11 and 17 pounds per minute. An initial application of avery thin layer of wet fiber appeared to promote adhesion between thesprayed-on ceramic fiber layer and the refractory brick. The furnacelining was built up to roughly 3 inch (7.5 cm) thickness in the arch forabout two-thirds of its length. At this point. the entire spray-appliedlayer on the arch fell. The fiber was knitted together but did not havemuch adherence to the ceiling or from fiber-to-fiber. The spray gun washeld at right angles to the surface to promote good bonding. Onexamination of the newly applied lining it was apparent that the fiberhad penetrated approximately 1/4 inch (6 mm) between the courses of thebrick.

Prior to spraying, two different anchoring systems were installed inportions of the arch to mechanically assist retention of the sprayed-onceramic fiber lining. The fumes of this binder were very objectionable.To protect themselves, workmen installing the lining should wear a fullsuit of protective clothing and a toxic fume-removing respirator system.

The first anchoring system tested included 28 inch (0.7 m) widthexpanded metal mesh having a diamond pattern of approximately 1/2 inch(12 mm) opening. The sprayed ceramic fiber/binder mixture did notadequately penetrate this mesh; rather, the mixture layed upon thesurface of the metal mesh. After building the sprayed lining to athickness of about 3 inches for about two thirds of the length of thearch the entire spray-applied layer of the arch fell. The fiber wasknitted together but did not have much adherence fiber-to-fiber.

The second anchoring system tested included metal mesh havingapproximately 3/4 inch-square (18 mm-square) openings. A second sprayattempt of the arch section including this second anchoring system hadalmost been finished when the whole arch lining fell again. The sprayedceramic fiber/binder penetrated these openings and filled the spacebetween the mesh and the refractory brick to which the mesh had beenfastened with powder-actuated fasteners.

A third spray attempt resulted in a lining about 2 inches (5 cm) thickwhich remained on the arch. It was concluded that about 2 inches (5 cm)thickness was the maximum attainable if fall-in was to be avoided.

Following successful spray installation of the lining, the furnace wasfired to 427° C. (800° F.) and thereafter cooled overnight. The liningremained in place and developed a very hard crusty surface. This crustysurface is believed due to migration of the binder to the hot facesurface of the furnace upon drying. Good adhesion appeared to existbetween the refractory wall surface and the fiber.

This installation was examined after approximately six weeks' use.during which it was cycled to a maximum of 927° C. (1700° F). The archof the furnace had lost its sprayed-on ceramic fiber veneer which hadoverlain the expanded metal mesh. The fiber was highly friablethroughout the entire thickness of the fallen sprayed layer. Thecondition of the fiber resembled that of 1260° C. (2300° F.) ratedalumino-silicate fiber prepared from kaolin clay that had been severelyover-fired. This degradation of the ceramic fiber is believed to becaused by the chemical nature of the binder.

EXAMPLE 2

A small laboratory test furnace having a single natural gas burner inits back wall and a flat steel lining defining its interior ceiling andwalls and a surface area of 180 sq ft was spray lined. A 6 inch (15 cm)thick lining was desired. Fiberfrax® 6000L bulk alumino-silicate fiberwhich had been lubricated during its manufacture by the application ofabout 4 percent by weight of an aqueous solution containing 50 HB-100polyalkylene oxide was used. A binder system was prepared using tapwater into which were introduced Natrasol® 250 HHR hydroxyethylcellulose and Ludox® HS-40 colloidal silica in sufficient amount toresult in respective binder solution concentrations of 0.3 and 10 weightpercent. The ceiling included Y-shaped anchors placed on 12 inch (30 cm)centers; no anchors were utilized in the walls.

Three attempts were made before a satisfactory lining was achieved.

Attempt Number one: binder solution delivery rate 18 lb/min, fiberdelivery rate 11 lb/min, Y-shaped anchors placed on 1 foot centers.Result was that ceiling fell-in when 6 inch (15 cm) thickness wasreached.

Attempt Number two: Binder solution delivery rate 18 lb/min, fiberdelivery rate 11 lb/min, anchors bent so that arms were parallel to andspaced about 1.5 inches (3.7 cm) from the ceiling. Result was thatceiling fell in when 6 inch (15 cm) thickness was reached.

Attempt Number three: Binder solution delivery rate 12 lb/min, Fiberdelivery rate 11 lb/min, anchors as in attempt two. Result was a liningof 6 inches (15 cm) thick which remained on the ceiling and walls.

Following installation of this spray-applied lining, the furnace wasfired to 1093° C. (2000° F.) for 24 hours and thereafter cooled and theshrinkage measured. Linear shrinkage was found to be approximately 1percent. Thereafter the furnace was again fired to 1093° C. (2000° F.)for 24 hours and cooled. The calculated shrinkage, based on initial andfinal measurements, was approximately 1.5 percent in the side walls and1.75 percent in the back wall. After firing a third time to 2000° F. for24 hours, shrinkage was determined to be approximately 2 percent basedon initial measurements. Thereafter the furnace was fired to 1177° C.(2150° F.) for 24 hours and thereafter to 1260° C. (2300° F.) for 24hours. The cumulative shrinkage at the conclusion of these cycles wasabout 2.7 percent in the side walls and 2.6 percent in the back wall.The lining remained in place with only a few small surface cracks whichdid not penetrate to the metal interior of the furnace shell. After thefiring cycles and measurements, the furnace was cleaned. The interiormetal surfaces did not show any evidence of corrosion attack by thebinder system. The ceramic fiber did not show any evidence ofdegradation. Good adhesion of ceramic fiber to the steel was observed.

EXAMPLE 3

The furnace of Example 2 was cleaned and relined with spray-appliedceramic fiber insulation. Cleaning was accomplished by cutting thelining into blocks, pulling out these blocks and thereafter sprayingwater on the shell and brushing with a broom to remove any remainingfiber. After cleaning, the furnace walls and ceiling were slightlyoxidized (Fe₂ O₃). An aqueous binder solution was made containing on adry weight basis as its organic binder 0.075 percent Reten® 525 and asits inorganic binder 10 percent silica from Ludox® HS 40. The ceiling,north wall and back wall of the furnace were sprayed using a Unisul™spray apparatus. The fiber employed was SEM Fiberfrax® 6000-L. Theceiling of this furnace was sprayed to form an approximately 2 to 3 inch(5 to 7.5 cm) thick base layer over the entire ceiling; thereafter, theceiling was gradually built up to a 6 inch (15 cm) thickness, doingapproximately 3 to 4 feet (1 to 1.3 m) sections at one time with theUnisul™ spray gun being held generally perpendicular to the surface. AUnisul™ Unifan™ nozzle was employed which included provision for 8binder delivery nozzles evenly arranged around the circumference of itsrace track-shaped fiber delivery opening. Six binder delivery nozzleswere employed with those nozzles which lie on the minor axis of theUnifan™ nozzle not being used. The north wall of the furnace was firstsprayed at its corners; thereafter, a 2-3 inch (5 to 7.5 mm) base layerwas applied which was thereafter built up to about 6 inches (15 cm)overall thickness, doing 3 by 4-foot sections at a time. The spraynozzle was maintained to 3-4 feet (1 to 1.3 m) distant from the surfacebeing sprayed. The back wall was also sprayed, first in its corners andthereafter around the burner, and completed using the same procedureemployed for the north wall to a thickness of about 6 inches (15 cm).Following deposition of the ceramic fiber and binder, this back wallsurface was further sprayed with binder solution only. During thistrial, the fiber feed rate was between 10 and 11 pounds per minute andbinder solution/dispersion was delivered at a rate between 13 and 131/2pounds per minute. Very little rebound was observed and was estimated tobe less than 1 percent. Prespraying of the corners appeared to create abracing force for the remainder of the lining to be sprayed adjacent thecorners. Gradual buildup of the lining to its final thickness appearedto be more effective than more rapid buildup of smaller areas of thefurnace. Spraying from a distance of 2 to 3 feet (0.6 to 1.0 m) resultedin a more uniform and flat lining of lower density than spraying fromcloser distances. Tamping of the wet newly applied lining resulted in aboard-like surface.

Following installation of this lining, the furnace was fired to 1093° C.(2000° F.) for 24 hours and thereafter to 1177° C. (2150° F.) for 144hours. After this cycle, the average linear shrinkage observed for thenorth wall was 1.2 percent based on initial measurements; the averagelinear shrinkage observed for the back wall was 0.8 percent based oninitial measurements.

EXAMPLE 4

The south wall surface of the furnace of Example 3 was insulated withsprayed-on fiber in a similar manner and at the same time that Example 3lining was installed (prior to any firing of the furnace) as follows:

A binder solution/dispersion was formed which contained on a dry weightbasis 0.075 percent Reten® 525 and 10 percent Nalco® 1034-A. The southwall of the furnace was sprayed utilizing the same machine settings forfiber and binder delivery rates as in Example 3. The south wall wassprayed in the same manner as the north wall except for the change ofbinder system. The doorway of the furnace was sprayed to form a 6 inch(15 cm) thick ceramic fiber insulation layer in the same manner as thenorth and south walls; thereafter, an additional inch was applied,resulting in a total thickness of about 7 inches (18 cm). The surface ofthe already insulated door was wetted with binder solution only and thentamped slightly to compact it resulting in a board-like surface. As inExample 3, the amount of rebound observed was less than 1 percent. Aftercycling the furnace, as in Example 3, linear shrinkage of the south wallwas determined to be an average of about 1.2 percent. No anchors wereemployed in the walls in either of Examples 3 and 4. The binder systemof Example 4 resulted in slightly lower measured linear shrinkage thanthat of Example 3. Fewer surface cracks were observed than in Example 3north wall. As in Example 3, good adhesion was observed, the liningremained in place until manually removed. Inspection of the metalsubstrate surface after removal of the sprayed material revealedevidence of slight corrosion in contrast to the north wall of Example 3which did not show any evidence of corrosion attack by the bindersystem.

EXAMPLE 5

Organic binder coated ceramic fibers were prepared by adding powderedwater-soluble polymers onto ceramic fibers as they were formed. Avertically-falling stream of molten ceramic material (alumino-silicatefrom kaolin clay) was blown into bulk ceramic fibers by impinging on ita high-velocity blast of air from a primary blowing air nozzle which wasdirected horizontally at right angles to the direction of fall of themolten ceramic stream. A twin-auger, K-Tron Corporation, model T-20volumetric feeder, was positioned approximately 3 inches (7.5 cm) aboveand 1 inch (2.5 cm) in front of the primary blowing air nozzle. Due toheat in the locale, it was necessary to protect the volumetric feederwith ceramic fiber insulation due to its proximity to the melt stream. Apolyalkylene oxide lubricant, Union Carbide Corporation 50 HB-100, wasapplied to the fiber as a 4 percent aqueous solution by introducing thissolution into the blast of air to provide a calculated lubricant levelof 0.5 percent based on dry weight of ceramic fiber.

Two powdered organic binder components were individually used to coatthe fibers: Natrasol® 250 HHR hydroxyethylcellulose from Hercules. Inc.,and Reten® 525 acrylamide polymer from Hercules, Inc. During ceramicfiber formation at 1300 lb/hr, one of these polymers was added at a timeat 3.5 lb/hr, 6.5 lb/hr, and 25 lb/hr, which by calculation correspondrespectively to coatings of 0.27 weight percent, 0.50 weight percent and1.92 weight percent based on dry weight of the ceramic fiber. Subsequentspray-on lining applications utilizing these fibers indicated that thehigher the organic binder component coating content the higher theadhesion and cohesion of the wet newly sprayed material.

EXAMPLE 6

The furnace of Example 3 was cleaned. Thereafter a new lining wassprayed using fibers which had been precoated with organic bindercomponent. The fiber utilized was that from Example 5 having bycalculation 0.50 weight percent of Reten® 525 acrylamide polymercoating. The liquid binder in this case consisted only of Ludox® HS-40colloidal silica (Du Pont) diluted to 10 weight percent solids withNiagara Falls, N.Y., tap water. The use of the organic precoated fiberresulted in an easier handling of materials. Less rebound was observedduring installation. Better wet adhesion and wet cohesion resulted thanwhen Reten® 525 is added to the binder liquid. Sprayed-on layers up to12 inch (30 cm) thickness were possible using this precoated fiber. Inthe present example, a successful installation was achieved of a 6 inch(15 cm) thick lining on the back wall, north wall and ceiling, and a 9inch (23 cm) thick lining on the south wall.

Following installation of this lining, the furnace was set at 427° C.(800° F.) for 8 hours and thereafter fired to 1093° C. (2000° F.) for 96hours to 1260° C. (2300° F.) for 120 hours. After this total firingcycle, the overall appearance of the sprayed material was excellent. Theaverage linear shrinkage was 1.6 percent.

EXAMPLE 7

An industrial furnace having an interior surface area of 860 square feetwas insulated by spraying uncoated bulk lubricated fiber (SEM 6000L).The binder system consisted of 10 weight percent silica solids derivedfrom Nalco® 1034-A colloidal silica (Nalco Chemical Co.) with 0.075weight percent of water soluble acrylamide polymer Reten® 525 in tapwater.

The flat ceiling (10 feet×30 feet) of the furnace was all steel with "Y"shaped anchors separated 6 inches (15 cm) between centers. The anchorswere all bent parallel to the ceiling. The side walls were lined withnew refractory brick. The back wall had 3 feet (1 m) of refractorylining on its bottom portion, the rest being a steel shell. The anchorswere relatively close to each other to investigate their effect on thesupport of the sprayed material on a relatively large area. No anchorswere utilized on the walls.

The liquid binder inorganic-organic system was pumped at a rate of 12-13lb/min while the fiber was delivered at a rate of 10-11 lb/min through aUnisul® A-300 apparatus. The ceiling was sprayed to 6 inches (15 cm)thickness in two passes while the walls were sprayed to 3 inches (7.5cm) thickness in a single pass. It was found that the 6 inch (15 cm)separation between anchors is not desired because they almost form acontinuous line. This made spraying more difficult because of the needto apply the wet fiber very carefully around the edges of the anchors toavoid the formation of voids. Better results are obtained when theanchors are separated approximately 12 inches (30 cm) between centers.After the successful installation of the lining. the furnace was set at427° C. (800° F.) overnight. The following day the furnace was heated toits usual temperature of 982° C. (1800° F.).

After 6 months of continuous cyclical operation, part of the sprayed-onlayer fell from the ceiling. The layer on the walls did not present anyproblems. Upon analysis of the fallen material, it was found thatproximity of the anchors created a continuous shear line whichcontributed to failure of the layer. It was also observed that a largeamount of rust had formed on the metal interface.

EXAMPLE 8

The flat steel ceiling of the furnace of Example 7 was relined byspraying the same uncoated bulk lubricated fiber used in Example 7 (SEM6000L). The binder system consisted of a Ludox® HS-40 colloidal silica(Du Pont), diluted 10 weight percent solids and 0.075 weight percent ofwater soluble acrylamide polymer Reten® 525 in tap water. Prior tospraying, the metallic Y-shaped anchors were rearranged to have 12 inch(30 cm) distance between centers instead of 6 inches (15 cm). The armsof the Y-shaped anchors were also bent parallel at 2 inches (5 cm) fromthe ceiling instead of 1 inch (2.5 cm).

The liquid binder system was pumped at a rate of 12-13 lb/min while thefiber was delivered at a rate of 10-11 lb/min. The ceiling was sprayedto 6 inches (15 cm) thickness. The larger separation of the anchorscenters facilitated installation around the edges of the anchors.Immediately after the installation was completed, the furnace was firedto 427° C. (800° F.) overnight. The following morning it was taken to1093° C. (2000° F.). Inspection of the lining after six months ofcontinuous thermal cycling to 982° C. (1800° F.) revealed the sprayedmaterial to be in excellent condition with only minor surface cracks.

EXAMPLE 9

An industrial car bottom annealing furnace of about 360 sq. ft. surfacearea on a refractory lined arched ceiling of about 16 feet (5 m) lengthand 121/2 feet (4 m) width was to receive 3 inches (7.5 cm) ofsprayed-on ceramic fiber insulation over its arched ceiling. Therefractory lining forming the ceiling was old, relatively uneven, andhad not been sand blasted. Its surface appeared to have a glassy-likefilm with noticeable carbon deposits apparent throughout its entirearea. Due to the production demands on this furnace, it was not possibleto delay installation by sand blasting the refractory. SEM 6000LFiberfrax® fiber was used. The binder was Nalco® 1034-A diluted to 10percent solids and 0.075 percent by weight of Reten® 525 in water.

The installation proceeded without problems. Immediately afterinstallation, the furnace was fired rapidly to 871° C. (1600° F.),although it was recognized that failure to allow the lining to dry at,for example, 427° C. (800° F.) might cause early failure. No anchorswere employed. After approximately one month of continuous usage, duringwhich the furnace was cycled to 1038° C. (1900° F.), about two thirds ofthe spray-applied layer of the ceiling fell.

EXAMPLE 10

The industrial furnace of Example 9 was reinsulated by spraying bulklubricated fiber. The fiber utilized was SEM Fiberfrax® 6000L. Theliquid binder in this case consisted of Ludox® HS-40 colloidal silicadiluted with tap water to 10 weight percent solids and 0.075 weightpercent solids Reten® 525 acrylamide polymer. The furnace was an oldrefractory unit having an arched ceiling. Some areas of the ceiling werein bad condition due to spalling of the bricks. No anchors were used.The binder was pumped at a rate of 12-13 lb/min while the fiber wasdelivered at 10-11 lb/min. The whole interior of the furnace wassuccessfully sprayed to 3 inches (7.5 cm) thick in a single attempt.After the installation of the lining, the furnace was set at 427° C.(800° F.) overnight. The following day the furnace was taken to 1038° C.(1900° F.) and cycled continuously to this temperature for approximatelyone month, after which part of the layer on the ceiling fell, indicatinga need for mechanical supports on overhead installations on thisrefractory surface. Failure was believed to be due to high thermalexpansion/contraction of the underlying refractory brick.

EXAMPLE 11

Different types of ceramic fiber were evaluated to determine theirrelative suitability for use in the formation of thermally insulatingcoatings by spray application in combination with binders utilizing aUnisul® 300 fiber sprayable machine. All of the fibers listed in Table Iwere prepared by Sohio Engineered Materials Company (SEM), FibersDivision, Niagara Falls, N.Y.

                  TABLE I                                                         ______________________________________                                                                Blown   Blown Blown 2                                 Comments      Spun Bulk 1       2     Lubed                                   ______________________________________                                        Sprayability  Poor      Okay    Good  Best                                    Avg. diameter (microns)                                                                     3.5       2.5     2.5   2.5                                     Length (cm)   up to 10  up to 5 up to 5                                                                             up to 5                                 Density (PCF) 7         12-14   8-11  8-11                                    Compatability with                                                                          No        Yes     Yes   Yes                                     Unisul ® equipment.                                                       Uniformity of sprayed-                                                                      Poor      Okay    Good  Best                                    on product.                                                                   ______________________________________                                    

Spun bulk fiber (prepared by pouring molten ceramic material onto cooledrapidly spinning wheel) does not work well with the Unisul® 300 fibersprayable machine unless the fiber is mechanically opened prior tofeeding to the spray gun. The Unisul® equipment does not provide suchopening action. Blown 1 fiber (prepared by pouring molten ceramicmaterial from a tilt furnace into a blast of air) processessatisfactorily through the Unisul® spray equipment but product densityand and amount of fiber used are excessive for many industrial furnaceinsulating linings. While Blown 2 bulk fiber (prepared under differentblowing conditions) is satisfactory, Blown 2 bulk lubed fiber ispreferred due to the presence of the lubricant which facilitatesprocessing through the spray apparatus. The lubricant also helps toprevent agglomeration (balling up) of the fibers. Agglomeration isundesirable because it inhibits effective coating of the fiber withbinder and results in a sprayed-on layer of non-uniform density.

EXAMPLE 12

The various binder candidates listed in Table II below were prepared bydiluting with tap water the concentrated sol as obtained from itsmanufacturer, and adding the powdered organic component. The liquidbinder was maintained at ambient room temperature of about 24° C. (75°F.) during and after preparation. A combination was rated compatible ifbinder candidate did not separate after mixing. Those binder candidateswhich did not separate upon standing were evaluated to determine theirpotential effectiveness. Evaluation was made using the laboratoryscreening procedure described hereinabove using an Instron® testingmachine set at 2 lb. full scale load, a 50 lb. load cell and a crossheadseparation speed of 0.2 in/min. The particular blanket product employedwas SEM Durablanket™ alumino-silicate fiber, of 1 inch (2.5 cm) nominalthickness and four (4) lb/cu.ft. (pcf) density. Sample size was 6 inches(15 cm) by 3 inches (7.5 cm) by 1 inch (2.5 cm). The 6 inch (15 cm) by 1inch (2.5 cm) area was wetted with the liquid binder sample andcontacted perpendicularly with the anchored plate.

                                      TABLE II                                    __________________________________________________________________________    Binder                        Average                                         Candidate                     Wet Adhesion                                    Sample No.                                                                          Descripton       Compatible?                                                                          (g/sq. cm)                                      __________________________________________________________________________    11-1  6.5% NALCO ® ISJ-614 +                                                                     No     --                                                    0.35% RETEN ® 525                                                   11-2  6.5% NALCO ® ISJ-614 +                                                                     Yes    6.7                                                   0.35% RETEN ® 420                                                   11-3  6.5% NALCO ® ISJ-614 +                                                                     Yes    5.4                                                   0.35% RETEN ® 210                                                   11-4  6.5% REMET ® COLLOIDAL                                                                     No     --                                                    ZrO.sub.2 (ACETATE SOL) +                                                     0.35% RETEN ® 525                                                   11-5  6.5% REMET ® COLLOIDAL                                                                     Yes    3.4                                                   ZrO.sub.2 (ACETATE SOL) +                                                     0.35% RETEN ® 420                                                   11-6  6.5% REMET ® COLLOIDAL                                                                     Yes    6.1                                                   ZrO.sub.2 (ACETATE SOL) +                                                     0.35% RETEN ® 210                                                   11-7  6.5% LUDOX ® HS-40 +                                                                       Yes    7.0                                                   0.35% RETEN ® 525                                                   11-8  6.5% LUDOX ® HS-40 +                                                                       Yes    5.2                                                   0.35% RETEN ® 420                                                   11-9  6.5% LUDOX ® HS-40 +                                                                       Yes    4.2                                                   0.35% RETEN ® 210                                                   __________________________________________________________________________

Each of binder candiates 11-2, 11-3, 11-5, 11-6, 11-7, 11-8 and 11-9 aredeemed suitable for use in sprayed-on ceramic fiber lining according tothe present invention. The higher the adhesion value, the better. It isto be noted that measured wet adhesion is a function not only of thetype of organic binder component but also the inorganic bindercomponent. For example, compare results for Examples 11-2 and 11-5.

Nalco® ISJ-614 is cationic (positive charge on particles) colloidalalumina. Remet® colloidal zirconia is cationic. Ludox® HS-40 is anionic(negative charge on particles). Reten® 525 is anionic polyacrylamide;Reten® 420 is non-ionic polyacrylamide; and Reten® 210 is cationicpolyacrylamide.

EXAMPLE 13

Base-type and acid-type aqueous silica sols were evaluated incombination with various organic polymers as to their effectiveness inobtaining wet adhesion. Evaluation was made using the laboratoryscreening procedure described hereinabove and in Example 11. The bindercandiates and the results obtained are listed in Table III below.

                                      TABLE III                                   __________________________________________________________________________    Binder                             Average                                    Candidate                          Wet Adhesion                               Sample No.                                                                          Description-Composition      (g/sq. cm)                                 __________________________________________________________________________    12-1  10% LUDOX ® HS-40 + 0.5% RETEN ® 523                                                               2.4                                        12-2  10% LUDOX ® HS-40 + 0.5% NATRASOL ® 250 HHXR                                                       4.0                                        12-3  10% LUDOX ® HS-40 + 0.5% CARBOPOL ® 941                                                            5.3                                        12-4  10% LUDOX ® HS-40 + 0.5% RETEN ® 525                                                               7.6                                        12-5  10% LUDOX ® HS-40 + 0.5% METHOCEL ® A4M                                                            7.1                                        12-6  10% NALCO ® 1034A + 0.5% NATRASOL ® 250 HR                                                         4.6                                        12-7  10% NALCO ® 1034A + 0.5% RETEN ® 525                                                               6.4                                        12-8  10% NALCO ® 1034A + 0.5% METHOCEL ® A4M                                                            8.2                                        12-9  10% NALCO ® 1034A + 0.5% RETEN ® 220                                                               11.0                                       __________________________________________________________________________

Each of Reten® 523, 525 and 220 is an acrylamide polymer.

The concentrations in Table III are expressed in terms of weight percentsolids. Tap water was used to dilute the concentrated sols. While eachof sample numbers 12-1 through 12-9 exhibited acceptable wet adhesionvalues, samples 12-3, 12-6 and 12-9 were deemed too viscous andgelatinous to be effectively sprayed onto the ceramic fibers. Theseorganic binder components should rather be precoated onto the ceramicfiber and used with a liquid binder having no or very low levels oforganic binder component in solution.

EXAMPLE 14

An industrial car bottom annealing furnace, having a brick refractorylining, was to be treated with sprayed-on ceramic fiber insulation. Thesprayable ceramic fiber lining utilized SEM Fiberfrax® 6000L, which hadbeen treated at its time of manufacture to have a coating of, bycalculation, 0.5 weight percent Reten® 525 based on weight of fiber. Thebinder consisted of Ludox® HS-40 colloidal silica diluted to obtain aconcentration of 10 percent solids by weight based on total weight ofbinder solution/dispersion. This trial was conducted during January inthe northern part of the United States. The brick surface of the furnacewas less than 5° C. (40° F.). The surface was relatively clean, havingbeen sandblasted. Expansion joint openings up to 1 inch thick werepresent in the ceiling of the furnace.

The first attempt to spray a ceramic fiber layer onto the ceiling of thefurnace was unsuccessful. It was observed that the sprayed-on materialhad less wet adhesion than usual. The furnace was warmed by firing threeof its seven burners for about 0.75 hour. Thereafter, no additionaladhesion problems occurred. Spraying was interrupted periodically torewarm the furnace until completion of the spray application. Based onthe poor performance observed at cold brick temperatures, it isrecommended that the substrate furnace be at least 10° C. (50° F.) atthe time of application.

EXAMPLE 15

A car bottom type furnace having a lining of dense insulating fire brick(IFB) and a surface area of about 180 sq. ft. was insulated withsprayed-on ceramic fiber insulation to a nominal thickness of 3 to 4inches (7.5 to 10 cm). Precoated fiber having 0.5 percent by weight offiber by calculation of Reten® 525 was used. Barbed studs 6 inches (15cm) long with twist-to-lock washers placed on 18 inch (45 cm) centerswere utilized in the flat refractory brick roof. The brick in thisfurnace was quite old and initially absorbed a large amount of bindersolution/dispersion (10 weight percent solids from Ludox® HS 40),resulting in separation of the sprayed-on ceramic fiber insulation fromits brick surface at first attempt. Heavy overspraying of this absorbentarea of the brick prior to simultaneous spraying of binder and fiberalleviated the problem. A portion of the roof had been repaired with aninsulating castable of unknown composition. This material was highlyporous and continued to absorb moisture from the binder, resulting inseparation of the sprayed-on ceramic fiber insulation even if presprayedwith binder solution/dispersion. Following installation of the lining,the lining was dried and cured by firing the furnace to a temperature ofabout 427° C. (800° F.) for about 8 hours. Thereafter, the furnace wastaken to its normal operating temperature of 982° C. (1800° F.). After 4months service, the lining remains in good condition.

EXAMPLE 16

A steel bell furnace lid which operates at temperatures between 871° C.(1600° F.) and 1038° C. (1900° F.) was totally relined with ceramicfiber sprayed-on insulation to a thickness of 6 inches (15 cm) using SEMFiberfrax® 6000L prelubricated fiber which had been precoated withorganic binder component Reten® 525 to a calculated level of 0.5 weightpercent based on dry weight of fiber. Unisul® spray apparatus wasutilized. The spray apparatus was adjusted to deliver the precoatedfiber at 12-13 lbs/min. and binder solution/dispersion at 12-13 lbs/min.The binder solution/dispersion was Ludox® HS-40 diluted with water tocontain 10 weight percent silica solids. This bell shaped movable coveris moved during normal furnace operation, thus subjecting the lining tolarge mechanical abuse. Ceramic spike anchors and Y-shaped steel anchorsplaced on 36 inch (0.9 m) centers were employed. After installation, thecover was allowed to remain stationary for 3 days to develop highinitial adhesion. The spray-applied layer appears in good conditionafter about 4 months continuous service.

EXAMPLE 17

The metal walls and ceiling of a steel shell industrial oven to be usedin curing of precast shapes at temperatures up to 1093° C. (2000° F.)were insulated with sprayed-on ceramic fiber to 6 inches (15 cm) nominalthickness. Approximately 850 lbs. of SEM Fiberfrax® 6000L lubricatedfiber, which had been precoated with 0.5 weight percent by calculationof Reten® 525 was used in combination with a binder solution/dispersionconsisting of 10 percent colloidal silica obtained by water dilution ofLudox® HS-40. Prior to spraying, the ceiling of the furnace was preparedby placing on 12 inch (30 cm) centers wiggle-legged V-type anchors.Identical anchors were placed on 18 to 24 inch (46 to 61 cm) centers inthe walls. No problems were encountered. After about 2 months inservice, the spray-applied layer remains in good condition.

EXAMPLE 18

To demonstrate the added wet adhesion and wet cohesion due to theorganic binder component, steel panels were suspended to present adownwardly facing horizontal overhead surface. Unisul® spray equipmentwas adjusted to deliver 11 lb/min of lubricated spun bulkalumino-silicate fiber and liquid binder solution/dispersion at 18lb/min.

The first panel was sprayed using as a binder liquid Ludox® HS-40diluted with tap water to 10 weight percent silica solids. No organicbinder component was used. When the sprayed ceramic fiber layer reacheda thickness of about 8.5 cm (3.5 inches) it fell from the panel.

The second panel was sprayed using as binder a solution/dispersion intap water of Ludox® HS-40 diluted to 10 weight percent solids andNatrasol® 250 HHR at 0.25 weight percent solids. The second panel wassprayed to a thickness of 10-11.5 cm (4 to 4.5 inches) without failure.Upon drying, the sprayed on fiber layer was firmly adhered to the panel.

We claim:
 1. A spray-applied refractory fiber thermally insulating layercomprising on a dry weight basis:(a) from about 72.5 to about 97.5weight percent ceramic fiber; (b) an organic binder component in amountsufficient to impart a wet adhesion to steel of at least 0.2 g/cm² ; and(c) an inorganic binder component in amount from about 2.5 to about 30weight percent based on weight of fiber of solids derived from acolloidal sol of a high temperature resistant refractory metal oxide. 2.The layer of claim 1 wherein the organic binder component is present inamount of about 0.05 to about 5.0 percent based on weight of ceramicfiber prior to heating of the layer above the decomposition temperatureof said organic component.
 3. The layer of claim 1 wherein the organicbinder component is selected from the group consisting of acrylamides,hydroxyethylcelluloses, carboxymethylcelluloses andcarboxyethylcelluloses, or mixtures thereof, and the sol is selectedfrom the group consisting of silica, alumina and zirconia, or mixturesthereof.
 4. The layer of claim 3 wherein the ceramic fibers include, asa coating thereon, from about 0.25 to about 1.0 percent by weight of thefiber of an organic polymer lubricant.
 5. The layer of claim 1 whereinthe organic binder component is present in amount of from about 0.25 toabout 2.0 percent by weight of the ceramic fiber and from about 5 toabout 15 percent by weight of solids derived from the colloidal sol ispresent by weight of the fiber, each of the binder components existingin the form of a coating on the ceramic fibers and at the contact pointsof the fibers.
 6. The layer of claim 5 wherein the organic bindercomponent is present in amount of from about 0.4 to about 1.0 percent byweight of the ceramic fiber.
 7. The layer of claim 1 wherein the organicbinder component is polyacrylamide and the inorganic binder component isderived from colloidal silica.
 8. The layer of claim 1 wherein theinorganic binder component is derived from base-type colloidal silica.9. The layer of claim 1 comprising from about 84 to about 95 percent byweight of ceramic fiber, from about 0.25 to about 2.0 percent by weightof organic binder selected from the group consisting of acrylamide,hydroxyethylcellulose, carboxymethylcellulose or carboxyethylcellulose,or mixtures thereof, and from about 5 to about 15 percent by weight ofsilica.
 10. A refractory fiber composition suitable for spray-appliedthermal insulation comprising:(a) dry ceramic fiber; (b) binderconsisting essentially of solution/dispersion containing 1.5-40 weightpercent solids of a high temperature refractory metal oxide or precursorthereof and from about 0.05 to about 2.0 percent weight of solids oforganic binder component.
 11. A sprayable ceramic fiber compositionsuitable for spray-applied thermal insulation comprising:(a) ceramicfiber having a coating thereon in amount of about 0.05 to about 5.0percent by calculation by weight of fiber of an organic polymer bindercomponent selected from the group consisting of acrylamides,hydroxyethylcelluloses, carboxymethylcelluloses, carboxyethylcelluloses,or mixtures thereof; and (b) a binder solution/dispersion consistingessentially of a sol of high temperature refractory metal oxidescontaining 5 to 30 weight percent solids.
 12. The sprayable fibercomposition of claim 11 wherein the ceramic fiber coating additionallyincludes an organic polymer lubricant in amount of 0.4 to 1.0 percent bycalculation by weight of ceramic fiber; and the organic polymer bindercomponent is polyacrylamide in amount of about 0.5 percent bycalculation.
 13. A method of forming a thermally insulating layer on asubstrate comprising:(a) providing ceramic fibers; (b) providing abinder including:(i) an organic polymeric binder component; (ii) aninorganic binder component in the form of a sol of a high temperaturerefractory metal oxide; (iii) a liquid vehicle which is a solvent forthe organic component and a diluent for the inorganic component; (c)Applying said sol and liquid vehicle to said ceramic fibers whilespraying said fibers toward the substrate.
 14. The method of claim 13further comprising forming a liquid inorganic-organic binder containingfrom 0.05 to 2.0 percent by weight of said organic binder component insaid liquid vehicle and 2.5 to 30 percent solids of said inorganicbinder component in said liquid vehicle; applying said liquidinorganic-organic binder in amount of 75 to 180 percent based on weightof ceramic fiber by spraying onto said fibers while they are in flight;the amount and concentration of said liquid inorganic-organic binderbeing sufficient to impart to said ceramic fibers on a dry basis fromabout 0.05 to 2.0 weight percent of said organic binder component and2.5 to 40 weight percent of high temperature refractory metal oxide. 15.The method of claim 13 further comprising precoating the ceramic fiberswith from 0.05 to 5.0 weight percent, based on fiber weight, of organicbinder selected from the group consisting of acrylamides,hydroxyethylcelluloses. carboxymethylcelluloses andcarboxyethylcelluloses, or mixtures thereof.
 16. The method of claim 15further comprising applying from about 75 to about 180 percent based onfiber weight of liquid binder to said fibers, said binder containingfrom about 0.05 to about 2.0 weight percent solids of an organic bindercomponent selected from the group consisting of acrylamides andhydroxycelluloses and from about 5 to about 15 weight percent solids inthe form of a sol of a high temperature refractory metal oxide selectedfrom the group consisting of silica, alumina, zirconia, or mixturesthereof.
 17. The method of claim 15 wherein said fibers are also coatedwith an organic polymeric lubricant.
 18. The method of claim 16 whereinsaid lubricant is polyalkylene oxide applied at between 0.4 and 1.0percent by calculation based on weight of fiber.
 19. The method of claim15 further comprising applying from 75 to 120 percent based on fiberweight of liquid binder to said fibers, said liquid binder containingfrom about 0.25 to about 1.0 percent by weight of solids of an organicbinder selected from the group consisting of acrylamide andhydroxyethylcellulose, and from about 5 to about 15 weight percentcolloidal solids of a high temperature refractory metal oxide.
 20. Themethod of claim 13 further comprising precoating the ceramic fibers byapplying thereto at the time of fiber formation, from about 0.4 to about1 percent by weight of ceramic fiber of polyacrylamide; and applying tosaid precoated fibers from about 75 to about 125 percent, based onweight of fibers, of a liquid binder including about 5 to about 15weight percent solids of high temperature refractory metal oxide. 21.The method of claim 13 wherein about 0.5 percent, based on weight ofceramic fiber, of polyacrylamide organic binder is applied to the fibersas they are formed, and an aqueous colloidal silica dispersion havingabout 10 percent by weight solids is applied to the fiber in amountapproximately equal to the weight of the fiber.